In recent, data usage over a wireless communication network is exponentially growing. Accordingly, beyond-4G mobile communication technologies are to support gigabyte communication even in the outdoors. For doing so, beamforming in a millimeter band is attracting attention as a candidate technology. The millimeter band can use a broadband, as compared to the existing lower-frequency bands. However, the millimeter band is subject to considerable channel attenuation. The beamforming has been studied to increase a signal strength in a particular direction using a plurality of antennas. As a wavelength is shortened in the high-frequency band, the multiple antennas can be integrated into a small region. Hence, the beamforming grabs the attention as a resolution for overcoming the channel attenuation of the millimeter band.
The beamforming can be implemented after a digital stage (e.g., after a baseband stage and Digital to Analog (D/A) conversion, or at a Radio Frequency (RF) stage). When the beamforming is performed at the digital stage, adjusting a beamforming coefficient (e.g., the signal strength, phase, and/or the like) is relatively easy. However, each antenna requires an RF chain leading to the baseband-D/A-RF, which complicates cost and configuration. By contrast, the beamforming at the RF stage lowers the cost but makes finely controlling the signal strength and phase relatively difficult.
Hybrid beamforming configures the baseband merely with a certain number of RF chains and connects an RF beamforming to each RF chain. Thus, the hybrid beamforming transmits multiple data via a digital precoder and obtains a beamforming gain through the RF beamformer. Thus, the hybrid beamforming can achieve an adequate trade-off, as compared to the digital beamforming and analog beamforming.
The hybrid beamforming can be combined with an Orthogonal Frequency Division Multiplexing (OFDM) system in diverse structures. For example, beamformers of the RF chains can be connected to a single physical array antenna via an adder. A Multiple Input Multiple Output (MIMO)-OFDM hybrid beamforming system can employ different digital precoders per subcarrier and different analog beamformers per RF path.
A MIMO-OFDM system according to the related art without the beamforming is subject to a high Peak-to-Average-Power Ratio (PAPR). To avoid a high PAPR, an operating point of a Power Amplifier (PA) (e.g., an average input signal strength) is lowered so that the input signal to the PA can be linearly amplified with a high probability, which is called PA back-off. In the MIMO-OFDM hybrid beamforming structure, signals as many as the RF paths are united by the adder and then input to the PA. Because the PA input signal strength can vary according to the number of the digital precoders and the RF beamformers and the transmit signal, further lowering the operating point of the PA may be necessary.
Mostly, based on hardware complexity, the same PA is used for antenna elements of the array antenna, and the back-off is fixed for the uniform transmission power in average. However, when the hybrid beamforming structure transmits the signal using multiple beams, signals are united and then input to the PA. As a result, an average transmission power input to the PA can differ according to the selected digital precoder, an analog beam combination, and the transmit signal. When such a transmission power difference is applied, a user in a particular direction receives the signal of great strength and thus a problem may arise in terms of fairness or selection criterion setting of user scheduling.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.